The disclosure relates to methods for predicting changes in characteristics of a near-wellbore area of an oil/gas bearing formation exposed to a penetrating mud (or cleaning fluids) during drilling, completion and cleaning wellbores.
Drilling muds are complex mixtures containing solid particles (having size from under one micron to few millimeters), clay and organic additives (polymers, starch, surfactants, etc.) suspended in a “carrying” fluid which forms a base of the mud: this can be water, oil, diesel fuel or synthetic fluid.
Overbalanced well drilling and completion can result in invasion of drilling mud components into a formation changing porosity and permeability properties, a multizone structure of a near-wellbore area is formed. Usually there are an external mudcake (which is formed at a borehole wall and consists of solid particles and clay filtered from the mud), an internal mudcake (formed by mud particles penetrating into formation rocks) and an invaded zone (a zone invaded by a drilling mud filtrate).
During clean-up jobs (bringing wellbores on production regime) components of a drilling mud are partially washed out from a near-wellbore area. Nevertheless, some components remain inside a pore space of a formation rock which results in degradation of properties of the near-wellbore area, first of all in reduction of permeability of the near-wellbore area. As a result, well productivity is reduced. To describe this phenomenon the term “damage of a near-wellbore zone” (or, for simplification, “formation damage”) is usually used.
The majority of mathematical models describing drilling mud invasion account only for formation/removal of an external mudcake and an invasion of a drilling mud filtrate (water/oil/diesel fuel) into reservoir rock; it is assumed that drilling mud particles do not penetrate into reservoir rocks.
Historically, a numerical simulation of dynamics of invasion of a drilling mud filtrate was used in methods for estimation of reservoir properties by comparing a calculated and apparent radial distribution of electric resistance obtained through multi-sonde; later results of simultaneous inversion of multi-sonde induction logging were used in combination with pressure dynamics obtained during downhole well sampling. These methods take into account a potential change in permeability and other properties of the bottomhole area resulting from invasion of drilling mud components only as an additional hydraulic resistance (associated with formation both internal and external mudcakes).
A more detailed workflow accounting for mechanisms affecting the properties of a near-wellbore area caused by penetrated drilling mud components is described, for example, in U.S. Pat. No. 7,099,811. The method is based on laboratory filtration experiments using long (40 cm) core samples; the experiments comprises injecting a drilling mud into a core sample followed by injection of a reservoir fluid from an opposite end of the core to simulate cleaning process. Profiles of damaged and repaired permeability were obtained along the core sample in laboratory filtration experiments; these results were used as input parameters for hydrodynamic simulation which takes accounts for distribution of permeability in the near-wellbore area using a cylindrical grid with small cells (of the order of a few millimeters) covering the area around the wellbore. Dynamics of change in permeability is modeled as a function of total volumetric flow through the core sample. The disadvantage of this approach deals with necessity to carry out a separate, sufficiently long, laboratory experiment to analyze effect of even relatively small variations in concentrations of drilling mud components or properties of original (“pure”) rock sample, or pressure drop during well drilling and clean-up jobs. This means that selecting optimum composition of a drilling mud, as well as finding and predicting an optimum drilling/clean-up regime for a wellbore requires a large number of laboratory experiments, especially when wellbores are drilled in a complex multizone formation.